1. Field of the Invention
This invention relates to adhesive glasses and more particularly to PbO--B.sub.2 O.sub.3 --Ag.sub.2 O glasses with low glass transformation temperatures (T.sub.g).
2. Description of the Previously Published Art
Current conductive die attach adhesives which are used for attaching integrated circuits to ceramic substrates are essentially a mixture of silver and glass powder in an organic vehicle. They require firing at temperatures in the range 420.degree.-450.degree. C. to achieve strong bonding so as to ensure reliability of the integrated circuit package assembly. The primary reason for requiring such firing temperatures is to achieve sufficient flow in the glass component of the adhesive to allow wetting of the integrated circuit chip and the ceramic substrate. These high firing temperatures can result in degradation of the integrated circuit chips and migration and oxidation of Ni on the metallization of the substrate. See, for example, Owens, N. L., Microelectronic Manuf. and Test., April 1986, pp. 6-7, at p. 7, which reviews nickel migration during organic burn-off.
To lower the firing temperature and still achieve good bonding, glasses having lower glass transformation temperatures (T.sub.g) or having more fluid properties at lower temperatures are required.
Most glasses used in conductive die-attach adhesives are PbO--B.sub.2 O.sub.3 based glasses with various oxide additives such as SiO.sub.2, ZnO, Bi.sub.2 O.sub.3, and BaO. These glasses have a T.sub.g typically above 300.degree. C. Alkali metal oxides (Na.sub.2 O, K.sub.2 O) and thallium oxide can be added to the glass melt to lower the T.sub.g, but such oxide additions are known to result in glasses having poor resistance to moisture and, therefore, not acceptable as die-attach adhesives. See Takamori, T., "Solder Glasses", a chapter at pages 173-255 in Treatise on Materials Science and Technology, Vol. 17, Glass II, Edit. M. Momozawa and R. H. Doremus, Academic Press (1979).
Certain other oxides lower the T.sub.g of PbO--B.sub.2 O.sub.3 glasses but are avoided for various reasons. CdO and BaO present additional toxicity problems; Bi.sub.2 O.sub.3 attacks platinum crucibles conventionally used in preparing the melts. PbO--B.sub.2 O.sub.3 based glasses will dissolve oxide crucibles such as silica, alumina, clay, porcelain, etc. It is therefore important to match the glass to the melt container and vice-versa.
To meet the stringent requirements for die attachment, aside from lower T.sub.g and considerations above mentioned, the glasses must also be resistant to crystallization during the firing step. This is because crystallization of the glass can lead to abrupt increases in viscosity and, hence, prevent flow and wetting of the integrated circuit and ceramic substrate.
Prior patents disclose various glass compositions, but they provide no suggestion of the low Tg PbO--B.sub.2 O.sub.3 --Ag.sub.2 O glass composition which are found effective here for die attach adhesives.
U.S. Pat. No. 3,711,328, Wada et al. (1973), discloses a glass frit consisting essentially of 0.1-17 weight % Ag.sub.2 O and 83-99.9% of a base composition consisting essentially of 60-80% PbO, 8-16% B.sub.2 O.sub.3, 12-24% ZnO. The basic glass composition has ZnO as a required component. In Example 12 of the patent, PbO is 64.7 weight %, B.sub.2 O.sub.3 11.6%, ZnO 13.7%, Al.sub.2 O.sub.3 7.0%, and Ag.sub.2 O 3.3%. It is used in making a resistor paste with no suggestion for use as an adhesive.
U.S. Pat. No. 4,401,765, Dietz et al. (1983), discloses a silver-filled lead borosilicate glass metallizing paste for bonding silicon dies to ceramic substrates. Ag.sub.2 O is absent.
U.S. Pat. No. 4,446,059, Eustice (1984) discloses a conductive composition containing lead borosilicate, Ag.sub.2 O, and Ag powder for use as automotive window defoggers. Proportions of ingredients for such composition are stated in column 3 of the patent as (with respect to the instant invention), 60-98.5% Ag, 20-1% lead borosilicate, and 5-60% of a blend of Ag.sub.2 O and B.sub.2 O.sub.3 as a colorant. This is a physical mixture of the ingredients and does not disclose the homogenous composition as set forth herein.
U.S. Pat. No. 4,636,254, Husson et al. (1987), discloses a silicon die attach paste consisting of a physical mixture of Ag flake 65.38 weight %, lead borate glass frit 17%, Ag.sub.2 O 2.62%, balance organics to make 100%. The Ag flake is 50% 200 mesh, 50% 100 mesh. We differ from Husson, inter alia, in that our Ag.sub.2 O is an integral component of our glass, whereas in Husson the Ag.sub.2 O is in physical mixture.
U.S. Pat. No. 4,699,888, Dumesnil et al. (1987), discloses adhesive glass pastes containing 75-85 weight % Ag powder and a glass containing PbO, B.sub.2 O.sub.3, SiO.sub.2, Cu.sub.2 O, a metal fluoride, and optionally ZnO, Bi.sub.2 O.sub.3. The materials were powdered, formulated as a paste, and used to bond silicon wafers to alumina substrates. Ag.sub.2 O was not disclosed. The pastes were fired at 380.degree. and 450.degree. C.